Its a hard question to answer with so little information in your post.

If you have applied to SSDI and reached the point of referral to a Psychological evaluation, just go to the appointment.

He/she will take a history, perform some initial neuropsychological tests which will help him/her decide if you need further Neuropsychological Assessment.

Depending on your application responses, and requests, they may have already decided on planned NeuroPsychological Assessments, and his/her initial evaluations are still performed, then you will proceed to the more involved testing. That testing will involve a few hours of tasking, questionnaires ,etc.

Bring a friend/family member/support if you need company, or help with cognitive things; ask if you can; that should not be a problem. If you stress easily, inform them when you feel so, and ask for a time out between tests if you need it.

Be honest, don't be afraid to be direct, don't hold your feelings inside, they are evaluating you for your problems.... they know when you are holding back. And the good ones know when you are putting on a 'front'.